Quick overview of core indicators:

750V 835V 1500V

Rated voltage (new breakthrough) Actual measured breakdown voltage (mean value) Anodizing forming voltage

Introduction: When electric vehicles need "stress-resistant" capabilities

When you climb into an electric vehicle and experience the powerful acceleration from a standstill to 100 kilometers per hour, there is an inconspicuous component that is enduring the continuous impact of hundreds of volts of high-voltage current, and it cannot tolerate any millisecond-level errors. This is the aluminum electrolytic capacitor - one of the key passive components in the traction inverter, whose weight can account for 20% of the entire inverter in some designs.  

Current pain point: The voltage rating of commercial polymer aluminum capacitors has long been limited to 100V to 200V. As electric vehicle platforms evolve from 400V to 800V, this voltage bottleneck has increasingly become a significant design flaw.

Until the team led by Tim Kruse from the University of Southern Denmark (SDU) published a paper that garnered industry attention. They successfully developed a polymer aluminum electrolytic capacitor with a rated voltage of up to 750V and a measured breakdown voltage of 835V, using only a thin piece of aluminum foil as the core material.

Part 1: Why is the capacitor the "heart partner" of electric vehicles?

In the process of electrification of complete vehicles, capacitors play a role akin to a "buffer pad". They are positioned between the battery and the motor drive system, responsible for stabilizing the DC-Link voltage and absorbing intense current fluctuations generated during rapid acceleration or energy recovery.  

1.1 Polymer aluminum capacitor vs traditional liquid capacitor

Traditional liquid electrolytic aluminum capacitors: high capacitance, low cost, but with the disadvantages of high equivalent series resistance (ESR), electrolyte evaporation, and susceptibility to failure after prolonged high-temperature use.

Polymer aluminum capacitors (replacing liquid electrolyte with PEDOT:PSS conductive polymer): ESR is significantly reduced, reliability is higher, and thermal stability is better. The disadvantage is that the upper voltage limit is low, with commercial products typically only reaching up to 100V, and barely reaching 200V at most.

As brands such as Porsche, Xiaopeng, and Hyundai shift to the 800V platform, designers often have to use multiple low-voltage capacitors in series to increase voltage tolerance. This comes at the cost of a surge in component count, the need for additional voltage balancing circuits, and increased system cost and complexity.  

"If there were a polymer aluminum capacitor capable of directly withstanding 750V or even 800V, the entire inverter design would be greatly simplified.". "

Part 2: Why is it difficult to break through traditional technology routes?

The theoretical approach to enhancing the voltage resistance of polymer aluminum capacitors is straightforward: increasing the thickness of the oxide film. However, in engineering practice, there exists a triple contradiction of mutual constraints.

① Thickening of oxide film → Decrease in packing density

The thicker the oxide layer, the lower the capacitance per unit area. To maintain the total capacitance, the area of the aluminum foil must be expanded, thereby increasing the device volume.

② Winding structure limits dimensions

Currently, mainstream polymer aluminum capacitors employ a winding process, and PEDOT:PSS needs to be impregnated after winding, which poses a high technological difficulty and limits the device diameter and height to approximately 1.2cm.

③ Absence of high-pressure etching process

Under low voltage conditions, tunnel structures can be formed on the surface of aluminum foil through chemical etching, significantly increasing the effective area. However, under anodic oxidation conditions above 1000V, thick oxide films can block the tunnel structures. Currently, there is no mature high-voltage etching solution in the industry.

Part 3: The Smart Strategy of the SDU Team - Turning "Rolling" into "Stacking"

The core breakthrough of Tim Kruse's team lies in the following approach: first, impregnation, then assembly, transitioning the structure from a wound-type to a laminated-type. The overall manufacturing process is divided into two key steps:

A High-voltage anodizing forming process (3-step cycle, 7-step method)

Using aluminum foil with a purity of 99.99% (150um thick), it undergoes electrochemical polishing first, followed by a "three-stage seven-step" oxidation process: ① First anodization: boric acid solution, 85°C, current density 3.75mA/cm², climbing all the way to 1500V; ② Chemical depolarization: diammonium hydrogen phosphate solution, 70°C, releasing trapped charges in the oxide layer; ③ Thermal depolarization: high-temperature baking at 500°C, allowing lattice defects to rearrange and improving the quality of the oxide film. The above three steps are repeated twice, ultimately completing the third anodization.

B. Fan-shaped laminated encapsulation structure

The cathode aluminum foil is cold-welded into a "fan-shaped" structure, with a paper separator layer sandwiched in the middle, and six anode aluminum foils inserted into it. The entire assembly is placed inside an aluminum case, directly impregnated with PEDOT:PSS inside the case, vacuum-treated, and then dried at 120°C. Finally, the leads are ultrasonically welded and sealed with polyurethane. Key innovation: The anode aluminum foils are kept in minimal contact, as the thick oxide film formed at 1500V is extremely fragile. Once it cracks, it will lead to early breakdown.

Part IV: Let the data speak - How does it perform?

2 μF 835 V 42 μA 43°C

@100Hz capacity  Breakdown voltage (mean value)  @750V leakage current  Temperature of the shell after heat dissipation

The breakdown voltage is 835V, which is 55% of the forming voltage of 1500V. Similar low-voltage products usually achieve 60-70% of the forming voltage - this is slightly lower. The researchers believe that the introduction of more defects due to the high-voltage oxidation process is the reason. Achieving a breakdown voltage of 1000V is an expected goal after future process optimization.

In the ripple current test (300V DC bias + 100V AC + 0.6A AC @ 500Hz), the temperature of the housing reached up to 75°C without heat dissipation; after connecting a heat sink, it dropped to 43°C, with uniform temperature distribution - fully demonstrating the inherent advantage of polymer aluminum capacitors in "active heat dissipation", which is a capability not possessed by liquid aluminum capacitors.

4.1 Frequency response characteristics

The capacitance is 2µF at 100Hz, decreasing as the frequency increases: it is 1.5µF at 10kHz and 1.1µF at 100kHz. The ESR decreases from 70Ω at 100Hz to 1.1Ω at 10kHz and 0.7Ω at 100kHz. Both the low capacitance value and high ESR are attributed to the use of flat aluminum foil (without etching for surface enhancement), which is the deliberate focus of this study, rather than a technical bottleneck.

[Figure 3 - Electrical Performance (Capacitance-ESR Frequency Response Curve)]

[Figure 1 - Schematic structural principle diagram of capacitor (laminated structure + packaging)]

Part 5: Horizontal Comparison - Its Position in the Jianghu

The research team compared the new product with two mainstream technologies of the same voltage level (750V, 2µF) available on the market:

Technology type  Rated voltage  ESR@10kHz  Capacitance density  Features

This study focuses on polymer aluminum capacitors, with a voltage rating of 750V (single unit), which is on the higher side. Currently, it is the lowest voltage rating available. It features heat dissipation and high reliability

Liquid aluminum electrolysis (×2 in series) 350~450V×2 Medium Highest (×54) Requires voltage equalization circuit · Limited lifespan

Metallized film capacitor, single unit 750V, minimum (×1/36), medium (×6), lowest ESR, no heat dissipation

key insight

When the effective area of anode aluminum foil doubles, the capacitance doubles and the ESR halves. This means that with only minor process improvements (introducing high-voltage etching for surface enhancement), the capacitance density of this technology can surpass metallized film capacitors, and the ESR will also be superior to liquid aluminum capacitors - truly occupying the "performance sweet spot" between the two.

[Figure 2 - Voltage Breakdown Comparison (Bar Graph of Breakdown Voltage for Various Technologies)]

[Figure 4 - Technical Comparison Radar Chart]

Part VI: In-depth Exploration - The True Value of This Study

On the surface, this is just a paper about a "experimental product" with a capacitance of only 2µF, which is far from commercial products. But those who understand it see three deeper meanings:

▶ Broken through the boundaries of technological cognition

Successful anodization of aluminum foil at a high voltage of 1500V was achieved, and a complete "three-stage, seven-step" molding process was established - this in itself represents a technological exploration unprecedented in the field of capacitors. Previous literature records only reached a maximum of 700V, while this study directly jumped to a molding voltage of 1500V.

▶ It has solved the packaging challenge and provided an engineering-friendly path

The solution of lamination + shell impregnation fundamentally circumvents the impregnation challenge of PEDOT:PSS in the winding structure, and can linearly expand capacity by increasing the number of laminations, with a clear path for engineering scale-up.

▶ Opens up new possibilities for the 800V electric vehicle platform

This study demonstrates that a single polymer aluminum capacitor is expected to be directly compatible with 800V systems, eliminating the need for series voltage balancing circuits, reducing system complexity, while retaining the high reliability and heat dissipation advantages of polymer aluminum capacitors.

�� The researcher's self-positioning is very honest:

They explicitly pointed out that the current low capacitance density is caused by the use of flat aluminum foil (without etching for surface enhancement), which is to "focus on high-voltage molding and packaging process verification". The next step of etching for surface enhancement process requires specialized independent research. This focusing strategy is actually a paradigm of excellent scientific research design.

thesis information

Authors: Tim Kruse*, Saykot Majumder, Luciana Tavares, Thomas Ebel

Institution: Department of Mechatronic Engineering, University of Southern Denmark (SDU), Sønderborg, Denmark

Contact the author: timkruse@sdu.dk | ORCID: 0009-0008-2884-4463

Publication platform: PCNS 2024 (Passive Components and Networks Symposium)